Now all our human insight is at an end as soon as we have arrived at fundamental powers or faculties, for the possibility of these cannot be understood by any means, and just as little should it be arbitrarily invented and assumed. Therefore, in the theoretic use of reason, it is experience alone that can justify us in assuming them. But this expedient of adducing empirical proofs, instead of a deduction from a priori sources of knowledge, is denied us here in respect to the pure practical faculty of reason. For whatever requires to draw the proof of its reality from experience must depend for the grounds of its possibility on principles of experience; and pure, yet practical, reason by its very notion cannot be regarded as such. Further, the moral law is given as a fact of pure reason of which we are a priori conscious, and which is apodeictically certain, though it be granted that in experience no example of its exact fulfilment can be found. Hence, the objective reality of the moral law cannot be proved by any deduction by any efforts of theoretical reason, whether speculative or empirically supported, and therefore, even if we renounced its apodeictic certainty, it could not be proved a posteriori by experience, and yet it is firmly established of itself. {BOOK_1|CHAPTER_1 ^paragraph 140} But instead of this vainly sought deduction of the moral principle, something else is found which was quite unexpected, namely, that this moral principle serves conversely as the principle of the deduction of an inscrutable faculty which no experience could prove, but of which speculative reason was compelled at least to assume the possibility (in order to find amongst its cosmological ideas the unconditioned in the chain of causality, so as not to contradict itself)- I mean the faculty of freedom. The moral law, which itself does not require a justification, proves not merely the possibility of freedom, but that it really belongs to beings who recognize this law as binding on themselves. The moral law is in fact a law of the causality of free agents and, therefore, of the possibility of a supersensible system of nature, just as the metaphysical law of events in the world of sense was a law of causality of the sensible system of nature; and it therefore determines what speculative philosophy was compelled to leave undetermined, namely, the law for a causality, the concept of which in the latter was only negative; and therefore for the first time gives this concept objective reality.